1. Field of the Invention
This invention relates to novel lubricant additives and the preparation thereof. More particularly, it relates to improvement in the viscosity index of lubricating oils by addition thereto of a certain class of ethylene copolymers which provide said lubricating oil with excellent low temperature viscometric properties relative to other ethylene copolymeric viscosity-index improvers.
2. Description of the Prior Art
As is well known to those skilled in the art, lubricating oils may be evaluated by many criteria each of which relates to the proposed use of the oil. One of the more important of these criteria is the viscosity index.
It is known that the viscosity index of lubricating oils can be usefully modified by the addition of viscosity index (V.I.) improvers such as polyisobutylenes, polymethacrylates, copolymers of polymethacrylate esters, copolymers of vinyl esters and fumaric acid esters, polyacrylates, polystyrenes, and terpolymers of styrene, methacrylate esters and nitrogen-containing esters and ethylene copolymers including ethylene-higher alpha-olefin copolymers and terpolymers.
Recently, ethylene-propylene copolymers have become widely used as viscosity improvers in lubricating oils because of the low treat levels and improved viscometric properties. However, the market requires different molecular weight grades, which have different degrees of thickening effect--usually called Thickening Efficiency or T.E.--so as to be operable with different viscosity mineral oils. Although the preparation of each of such copolymer grades can be by direct synthesis, the different molecular weight grades can be produced by degradation of an ethylene-propylene copolymer so as to produce lower molecular weight versions. It is advantageous to use the degradation approach since it is more economical to make a large run of copolymer in a large scale polymer plant, and then to use this run as a base material which is then broken down into lower molecular weight grades in order to meet requirements of the viscosity index improver mark. There are various means to achieve such degradation including: heating an amorphous rubbery ethylene-propylene copolymer for 3 to 30 minutes at from 260.degree. to 420.degree. C. with the exclusion of air (see United Kingdom Pat. No. 1,001,455); extruding and heating an ethylene copolymer first at 150.degree.-280.degree. C. until molten and then at a temperature of 300.degree.-500.degree. C. with the exclusion of air (see Canadian Pat. No. 991,792); and, by oxidative and mechanical degradation, as by mastication of ethylene-propylene copolymers preferably free of other monomers such as dienes (see U.S. Pat. No. 3,769,216). It is known that ethylene terpolymers which contain dienes, e.g. vinyl norbornene, are not suitable for mechanical degradation as by mastication in the presence of air or oxygen whereby oxidation occurs since this degradation technique produces excessive amounts of gel particles which are oil insoluble.
Frequently, it is also found that the presence of ethylene copolymeric V.I. improvers in conventionally-formulated lubricating oils undesirably raise the low temperature pour point of a lubricating oil containing a pour point depressant. This appears to occur because these copolymeric V.I. improvers interfere with the operation of the lubricating oil pour point depressants. Representative of these copolymeric V.I. improvers that are frequently incompatible with pour depressants are ethylene-propylene copolymers containing 60-80 mole percent of ethylene (see U.S. Pat. No. 3,697,429).
One approach to overcoming this incompatibility problem is taught in U.S. Pat. No. 3,697,429 wherein the V.I. improver is a mixture of two ethylene copolymers, i.e. a first copolymer of ethylene and a C.sub.3 to C.sub.18 higher alpha-olefin having an ethylene content of 50-95 mole percent (40-83 wt. percent) and a second copolymer ethylene and a C.sub.3 to C.sub.18 higher alpha-olefin having an ethylene content of 5-80 mole percent (3-70 weight percent) with the ethylene content of said first copolymer being at least 5 mole percent (4 wt.%) more than the ethylene content of said second copolymer. These ethylene copolymers can include diolefins containing about 6-28 carbon atoms, e.g. 5-vinyl-2-norbornene, as a third monomer. The ethylene copolymers of the mixture are chosen so as to provide a weight average ethylene content comparable to the characteristic modal weight percent at which the pour point is at a maximum for the particular lubricating oil.
It is an object of this invention to provide an improved ethylene copolymer viscosity index improver, preferably one that can be readily degraded to a lower molecular weight.